Electrical conductor



Patented Dec. 12, 1944 ELECTRICAL CONDUCTOR Donald Finlayson, ErnestLeslie Greenwood, and William John Simpson, Spondon, near .Derby,England, assignors to British Celanese Limited, London, England, acompany of Great Britain No Drawing. Application August 4, 1942, SerialNo. 453,558. In Great Britain July 7, 1941 3 Claims.

This invention relates to electrical conductors and is particularlyconcerned with the insulation of wires for this purpose.

The invention is primarily concerned with the insulation of very finewires, particularly of the order of 33 to 50 S. W. G. (British standardwire gauge) corresponding to wire diameters of 0.010" to 0.001" andapproximately to 29 to 40 American wire gauge (B and S), although ofcourse the present invention may be applied to wires of coarser gauge.

For the insulation of such wires, two standard methods are available;firstly the wire may be lapped or wound with a covering of silkfilaments, and secondly the wire maybe covered with a coating of varnishor enamel. The latter methodis subjectto the possibility of minutepinholes which render it unreliable, presents difiiculties in obtaininguniformity of coating owing to a tendency of the applied material to runin the liquid state to one side of the wire or along the length of thewire, and further cannot always be used in conditions where the wire issubjected for long periods to even moderately. high temperatures. Themethod of lapping with a covering of silk filaments is expensive onaccount of the high price of silk, and, moreover, if the silk lapping isdamaged locally the filaments may run back so that the insulation isimpaired beyond the point at which damage occurs. Instead of silkfilaments, the continuous artificial filaments, e. g. filaments ofcellulose acetate, have been used for this purpose, and although theyare less expensive than silk they are subject to the other disadvantagesof silk mentioned above. In addition, the only materials of this kindavailable on a commercial scale have been of a denier of the order of2.5 and upwards which gives them a lower covering power than therelatively finer silk filaments. Again, both with silk and withartificial silk filaments of the deniermentioned, the tension applied tothe filaments in the course of the lapping operation is such that thefilaments tend to cut into the very fine wires employed, or at least toimpose upon them. such a strain as causes the wire to be forced from astraight configuration into a slightly helical form.

It has been proposed to cover wires with artificial filaments of organicderivatives of cellulose such as cellulose acetate and '-to coalesce thefilaments partially or completely in order to impart a continuous andrelatively waterproof surface to the layer or layers of filaments afterthey have been lapped upon the wire. So far as we while this lowextensibility maybe increased by 1 are aware, however, this methodhasmet-with no commercial success, presumably. on account of the factmentioned above that the filaments commercially available were oftooxcoarse a denier.

Further investigation and research. into this problem have shown thatfine wires can be efficiently insulated by lapping them with artificialfilaments of a thermoplasticmaterialj having electrically insulatingproperties, pro.- vided that filaments of lower denier, than thoseheretofore available are used and that the filaments arecoalesced toform sulating cover for the wire.. w 7 I While filaments, of a denier.of less than 1.5 and if desired very much less, e. g.;of the .order of0.5 denieror less, may be produced b stretching coarser filaments of athermoplastic material in the presence of a softening agent totwice, orfive, ten, or more times their original length, the filaments produc din this way have a very much reduced extensibility whichgives rise to Idifiiculties in the lapping operation, Moreover,

at subjecting the stretched yarn to a shrinking opthe filaments to there eration, such a: step involves increasing unduly the cost of thefilamentswhich isalready high by reason of the cost of the stretchingstep'itself. For these reasons it. is preferred to spin uired low denier:in the first instance; i. e. in th course of theoperation of formingthe filaments by extruding a spinning solution through fine orifices.been found, is. made possible in the case of cellulose acetate filamentsby the use of a spinning solution of low concentration and-consequentlyof low viscosity. The fine artificial filamentsemployed for the purposeof the present invention maybe of any filament-formingorganic-substance. having ahigh electrical resistance and;capable ofbeing softened to bring about coalescence without impairing theinsulating propertiesof the substance of the filaments. As previously,mentioned, a suitable material for use as the basis of the filaments iscellulose acetate. Other organic derivatives of cellulose maybe employedsuch as other cellulose esters, e. g. cellulose propionate and butyrate,mixed esters such ascellulose acetatepropionate and acetate-butyrate,"or" cellulose ethers such as ethyl and benzyl cellulose. In the lappingoperation the filaments "are preferably employed in the form of athreador bundle having at most very little twist andpreferably no twist atall. The difficulty of collecta continuous in- I This, it has in thelapping operation. Thus, the filaments wound off the swift abovementioned may be wound into cheeses of, say, from %"-2 in length,internal diameter, and 1%" 2" external diameter, according to therequirements of the machine, on which'lapping isto take place. By thesemeans it is [possible to effect the spinning and re-winding of filamentshaving a denier of unity or less, in the form of bundles having a totaldenier of 22 or less,

1 though for most purposes filament bundlesof from 30 to 150 denier arerequired. The spinning and winding operation is facilitated by theapplication of a suitable lubricant tothe bundle,

an example of which is a solution containing 20%"by weight oftriethanolamine oleate, 5%

. oleic'acid and 75% mineral oil.

By the use of a'twistless bundle of fine m ments produced in the mannerdescribed above. wires' can be lapped by the methods employed in thesilk lapping of wires, though care is necessary if the operation is tobe conducted at the same -'speed as is employed for silk lapping. Forexample, it is desirable to eliminate, in the guiding surfaces of thelapping machine, roughnesses that might be tolerable in connection withsilk, and, further, it is generally better to use a lower tension'in thefilament bundle than is the practice with'silk. The necessityof thelast-mentioned precaution'is' not wholly disadvantageous, since iteliminates the defects sometimes experienced in silk lapping, due to thetension in the silk "reacting after lapping, on the wire itself, forcingit outof the straight into a slightly helical shape.

Apart from these points, however, the invention enables certainadvantages to be obtained by de-v parting from the methods ordinarilyemployed in 1 silk lapping. The size of the bundle employed in theordinary practice of silk lapping wires of from 18 to 48 gauge is of theorder of 30-100 denier.

' The denier is roughly proportional to the diameter of the wire, inorder to make the angle between the filaments and the axis of the wireapp'oximately the same in all cases-of the order of. 45. Reduction ofthis angle would enable a heavier bundle to'be employed, and anincreased After lapping the wire the filaments wound on the surfacethereof are caused to coalesce. This is preferably effected by. theapplication of heat, which may be employed either alone or with theassistance of a medium or high-boiling solvent or plasticiser to reducethe temperature necessary to bring about coalescence. If heatis employedalone to effect coalescence,- thedegree of coalescence efiected may besufiicient to bring about an appreciable fusion of the filaments so thatthey form a transparent or semi-transparent covering of uniformthickness through which the wire can be seen. If coalescence iseifectedwith the aid of a mediumor high-boiling solvent or plas-, ticiser, alower degree of coalescence may be effected, which allows thefilamentary structure to remain in some degree, andgives a coveringwhich is opaque or. of dull lustre.

The application of heat to the wire after lapping may be effected in'anenamelling furnace of the kind used for enamelling fine wires for use aselectrical c0nd110tors,'the w'lres being supplied from a creel carrying-.a-number of. supply packages and. after passing through the furnace,returning to the creel tobe rewound. Such'a furnace may be used withoutsubstantial modification, unless it is desired, on account of the natureof the insulating material employed and its behaviour at hightemperature in the presence of air, to provide for the heating to takeplace in an inert gas. In such circumstances, appropriate provision maybe made for the supply of inert gas to the furnace, and for preventing.the gasfrom escaping unduly rapidly through the openings by which thewires enter and leave the furnace.

Where cellulose acetate filaments are employed, a temperature of from250 C.-320 C. is appropriate'a'nd a time of l-25'secohds, according tothe temperature employed and the degree of coalescence required.Thus,rby running a wire of 28 standard wire gauge; 'covered witha doublelapping of a 100 denier bundle of cellulose acetate filaments, at 9 ft.aminute through a furnacetube 40" in length at a temperature of 300 C. awell coalesced transparentlacquer-like covering is obtained, while witha temperature as low as 250 C.the filamentary structure is quite clearlyvisible. Again, good-results may be obtained by exposing the lapped wirein a furnace at 290-300" C. for a period of 14 seconds, or at 305 C. fora period as short as 4 /2 seconds. Instead of a single run through theenamelling furnace, the lapped wire may be given two or more ms, athigher speeds so as to give a shorter exposure in each run than would berequired in asinele run.

rateof production to be obtained, but is undesirable as resulting in alayer of insulation that is liable to slip along the length of the wireand leave gaps in the insulation. By the use of the present invention,however, this disadvantage may be overcome, since. after coalescence,the

' filaments employed are bonded together and, to

the wire itself, and have little or some extent, to no tendency to slip.In consequence the invention enables lapping yarns of larger-deniers tobe employed and to be disposed atasrnaller angle to the axis of thewire,so that the rate of production may be correspondingly increased. For

the purposes of the present invention, as in silk 7 lapping, a singlelayer of filaments may be laid on the wire in the lapping operation, ortwo or even more layers; according to the gauge of the wire and thetotalthickness of insulation permissible.

. furnace, other means may be employed for the purpose of applying heatto the lapped wire.

Instead of using a standard type of enamelling Thus, the wire may beheated by radiant heat emitted by infra-redlamps, the radiation beingsuitably focussed on the line of travel of-the wire being heated. Forthis purpose a reflector in the form of a cylinder of elliptical crosssection may be employed, the filament of the infra-red lamp being at onefocus of the elliptical section and the ath of the wire being at'theother.

In order to reduce the temperature necessary to effect coalescence, ahigh-boiling solvent or plasticiser may be employed. High-boiling 1plasticisers, such as dlcresyl glyceryl ether or the ace-- tate thereof,or diethyl phthalate, ortriacetin,

may be employed in quantities up to 5% of the weight of the yarn or evenmore, the plasticiser being applied by running the lapped wire through asolution of the plasticiser, e. g. in methylated spirit, the methylatedspirit being evaporated by passing the wire through warm air at about100 C., thus leaving the plasticiser in the filaments, It is preferable,however, if a liquid agent is to be employed, that it should be 'alowmethylated spirit is readily evaporated, but the diacetone alcoholevaporates much more slowly, and has the effect, in conjunction'with thetemperature to which the wire is exposed, of coalescing the filamentsthereon. As an alternative to. applying the low-boiling plasticiser tothe lapped wire, the wire itself may be run through the liquid to beapplied immediately prior to lapping and continuously with thatoperation. Again,

the application of heat may be effected continuously with the lappingoperation, a suitable length of run and temperature being employedcorresponding with rate of travel of the yarn as it is lapped.

While, as stated above, it is preferred to employ heat for coalescingthe filaments, coalescence can be brought about by relying wholly on theaction of a softening agent for the substance of the filaments, theagent being applied to the filaments by running the lapped wire througha bath of the agent. Where cellulose acetate filaments are employed,convenient softening agents for use alone are acetone or methyl ethylketone,

though any other of the well-known softening agents or plasticisers forcellulose acetate may be employed, provided that a time of immersion isallowed appropriate to the softening power of the agent used. Diluents,such as benzene or alcohol, may be employed to modify the severity ofthe solvent employed, or, if desired, a solution in low concentrationsay -2%, of the substance of the filaments may be used. After havingpassed through the softening agent, the emerging wire may be dried byallowing a convenient length of run in air which may take place in atube or chamber from which the air is removed for subsequent solventrecovery. The product then resulting is an electrical conductor havingan insulating covering intermediate in form between a filamentarylapping and a layer of varnish or enamel, the filamentary nature andstructure of the covering being to some extent retained whiletheadjacent turns of filament in the lapping are coalesced and caused tounite with one another to form a covering that is continuous along thelength of the wire.

The filaments employed for the purposes of the present invention may bedyed or otherwise given a distinctive appearance, e. g. for the purposeof distinguishing from one another, wires intended for differentpurposes, or for identifying different wires assembled in the form of acable. Care should be taken, however, that the methods and materialsused for this purpose should .not be such as to impair the insulatingproperties of the filaments, and that the distinctive effect, ifrequired in the finished product, should not be destroyed by the step ofcoalescing the filaments,

e. g. by the application of heat. Dyes may be applied to the wires afterthey have beenlapped with the filaments, in order to colour thefilaments, and this may be done in some circumstances by incorporating asuitable dye in a solvent or softening agent that is to be usedto effector facilitate coalescence of the filaments.

Thus, where a mixture of equal parts byvolume of triacetin andmethylated spirit isusedas a solvent or softening agent, a red colourmaybe given to the filaments by addingto the liquid 1.25 grams per litreof alpha-ethanolamino-anthraquinone, or a blue colour by adding the sameproportion of 1:4-di(methylamino) -anthraquinone, or a yellow colour byadding the same proportion of 4-acetylamino-2'hydroxy iphenylazobenzene.Other colours maybe obtained by using mixtures of the above dyes.Pigmented filaments, obtained by incorporating a finely divided pigmentsuch as titanium dioxide in the spinning solution from which thefilaments are formed, may be used for the purpose of the pres sentinvention, and may be especially convenient when a thorugh coalescenceby means of heat alone is effected, whereby unpigmented filaments wouldform a transparent coating and the insulated wire might be mistaken atsight for bare wire. I t

The method of insulating according, toth invention may be employedbyitself, especiallyon "very fine wires of from 47-50 gauge, or in othercases where very thin layers of insulation are required. The method mayalso, however, be employed in conjunction with other methods ofinsulating. For example, where normally, a double lapping of silk mightbe employed, as on wires of the order of from 30 to 46 gauge, the innerlayer of silk may be replaced by a lapping of filamentary materialscoalesced in accordance with the present invention and an outer lappingof silk may be superimposed on the inner layer so provided. Again, aninsulating covering may be superposed, by the method according to theinvention, on a coating of enamel previously applied.

By means of the invention, electrical conductors, and especially finewires of the order of 30-35 gauge, can be insulated very satisfactorily.Among the advantages that may be obtained in conductors insulated inaccordance with the invention are the previously mentioned advantage ofavoiding slipping of a filamentary layer of insulating material alongthe length of the wire so as to leave bare places, and the relativelylow cost, as compared with silk, of the filamentary materials that areemployed. At the same time, materials may be. selected that giveinsulating layers of which the desirable electrical qualities,

particularly the insulation resistance and breakdown value in voltageper unit thickness, are

equal, and even superior to those of silk coverings.

Advantage may be taken of this fact, and also of the fact that filamentsas fine as, and even finer than, silk may be used, to obtain a productin which the coverings are thinner than silk coverings having the sameresistance and breakdown The insulating material being initiallydistributed in a solid state uniformly round the wire, and thesubsequently induced fluidity necessary to bring Some ample of themanner in which .maybe carried'out:v

about coalescence being only slight; in degree and not long maintained,the material has neither a great tendency nor a sufiicient time to flowunder gravity towards one side of the wire, as is apt tooccur when wiresare insulated by .enamelling. The initialuniformity of the coating mayconsequently be retained during coalescence and in the final product.Another importantproperty thatrmaybeimpartedlto the insulating coveringby meansof the invention is resistance to me- 'chanical abrasion. Inthis respect again, the advantage is best obtainable by the use of heat,and

preferably "heat only, as the softening agency,

the degree of abrasion resistanc depending upon the temperature and timeof exposure with which coalescence is efiected. The, following is anexthe invention Example. j

Copper wirefof 36 s. W. G. (.0076 inch diameter) lapped with a singlecovering of cellulose acetate yarn of 50 denier' containing 50filaments,

waspassed once at a speed of 54 feet per minute through a furnace tube 4feet long, he ated to 305 C., v

of the advantages mentionedabove are illustrated by the followingcomparison Acovered wire A prepared as described in the examplewascompared by standard methods with fajBB S: W. G. wire B having a doublecovering of 'silk. Tlie abrasion resistance was compared in two ways; byrubbing the wires in one case (a) witha 'polished round rod'of 1diameter, and

I V E Wire A Wire B Thickness of covering in mi1s i (i. 6 2. Insulationresistance in kilomeg'ohms 75 75 Breakdown v0lts 425 450 "Breakdownvalue in volts per mil 708 166 'Abrssion resistance, number of rubs.;ggg 22g 'iri-th-othr (b) witha'knife edge of 0.015"

radius'j' the wire in each case being tensioned with a load of 100grams. The following results were obtained: a

- It will be seen that, r iotwithstandin gthe comparative thinness ofthe covering of the wire A insulated in accordance vwith the presentinvent ion,'its insulation resistance is the same as the v greater,

wire B and the breakdown voltage scarcely less, the breakdown valueinvolts per mil. being much Again, the abrasion resistance asnieasuredbyamoderatelyv severe abrasive action is actually greater forfthe wire Athan for thewire B, and as measured by a more severe-abrasive action isstill of the same, orderjinspite of .the

smaller thickness of material.

Having described our invention-what we desire to secure by LettersPatentis:

as an elec- 1. Method of insulating wiretor use trical conductor, saidmethod comprising lapping the wire with ,fine artificialiiilamentshaving a denier not greater than ,1.5 and'consist'ing of a thermoplasticmaterial having electrically ins ulating properties, applying to thelapped wire a liquid softening agent for the substance oi, thefilaments, and then subjecting the wiifato; the

, action of heat so as to cause the filaments on the wire to coalesceand form a continuousins lating coverforthe wire.

', the wire with 2. Method of insulating wire forvuse as an'l'electricalconductor, said method comprising lapping fine artificial. filamentshaving a denier not greater, than 1.5 and consisting of a thermoplasticmaterial having electrically. in-

sulating properties, applying to .the lapped wire a liquid containing ahigh boiling solvent or plasticiser for the substance of thefilaments,'and then subjecting the wireto the actionoi heat so as tocause the filaments on the wireto coalesce and form a continuousinsulatingcover for the wire.

v 3. Methodof'insulating \tire,, for use as an electrical conductor,said i'nethod comprising lapping the wire, with fineartificialffilaments having a denier not greater than 1.5 'andconsisting' of a thermoplastic material having elec-' tricallyinsulating properties, applying to the lapped wire a liquid softeningagent for thejsubstance of the filaments, subjecting the wifre'to'theactionof heat so as to cause the filamentsfon the wire to coalesce andform a contihuous insulating cover for the wirey andthenlappi' i f theinsulated wire with 'an additional coating of' 'nonthermoplasticfilamentary inaterial;

5 DONALD' FINLAY'SONQ ERNEST LESLIE GREENWOOD. WILLIAM JOHN s MPsoN,

